Exploration companies need oil prices of $70 a barrel to match the returns they made at $30 a barrel just two years ago because of the sharp increase in costs and higher government licence fees, according to analysis by a leading consultancy. The research, from Wood Mackenzie, the Edinburgh-based oil consultants, helps explain why non-Opec oil production is failing to accelerate its annual growth significantly in spite of record prices. Oil prices have been above $70 a barrel only since September.